For Splinterbark I chose Pelagos again as he feels the strongest. (He might not be for this one)
Splinterbark is a fast melee treant with slow powerful attacks. He has two phases as well.
Phase 1 (end at roughly 60% hp):
Splinterbark will chase you around the arena while instantly casting splinters from his body landing on the ground around the arena. These are extremely easy to dodge, I actually didnt get hit by one so I do not know the effects if they hit you. During this phase you can use Pelago's second ability Aspirant's Bindings to root Splinterbark in place.
Phase 2 (enrage phase):
Splinterbark will become enraged and will deal huge damage if he hits you. Each hit will be roughly 25% of your life, so do your best to avoid being hit. Additionally, enraged splinterbark means he cannot be snared by Aspirant's Bindings. This was scary to figure out at first, but the binding will still deal damage. The enrage stated that it would go away when Splinterbark reached a set amount of energy, but I chose to avoid getting hit enough for him to do so. Furthermore, enraged Splinterbark will not cast the splinter AoE, rather he is just content chasing you around the arena trying to melee you to death.
Throughout both phases I used hit and run tactics for the ENTIRE fight. Only in phase one when you can root Splinterbark did I dare cast Pelago's first ability Test of Faith. I barely used it during phase two and that was when I got hit by Splinterbark's empowered melee attack. It is VERY important to use Pelago's second ability (Confront Memories) on cooldown. Always choose the left option, Swift Recollection, to receive a movement speed buff for 15 seconds. During this time constantly run from Splinterbark while facing him to make sure your auto attacks keep going off. Focus on keeping good distance while maintaining Swift Recollection and using Aspirant's Bindings for extra damage.
The fight was not super hard overall. I am sure there are more mechanics that I did not explore. However I feel using the above strategy worked great and I wouldnt be surprised if you ended the fight without taking any damage.